1. Field of the Invention
The present invention relates to a process for the preparation of acetic anhydride by the carbonylation of methyl acetate.
2. Description of the Prior Art
It is well known that acetic anhydride can be produced by the carbonylation of methyl acetate under relatively severe pressure conditions, in the presence of nickel complexes of the formula EQU [A.sub.4 M].sub.2 NiX.sub.4
in which X represents a bromine or iodine atom, M represents a phosphorous or nitrogen atom and A is, for example, a lower alkyl radical; compare U.S. Pat. No. 2,729,651. These complexes, which are obtained by reacting nickel halides with quaternary phosphonium or ammonium halides, can be used in this form in the reaction in question, or alternatively they can be formed in situ. However, the efficiency of this type of process is low, despite the high pressures used.
More recently, catalyst systems have been proposed which make it possible to carbonylate methyl acetate under less severe pressure conditions. Thus, U.S. Pat. No. 4,002,678 describes the carbonylation of methyl acetate in the presence of nickel, chromium, iodine (or an iodine compound) and a phosphine (or an amine), under a pressure of less than 70 bars.
At the same time, it has been shown that chromium is not necessary in a process of this type if the reaction is carried out in an aliphatic carboxylic acid as the solvent, and provided that the iodine (or the iodine compound) is used in the reaction in a proportion such that the fraction of iodine which is not chemically bonded either to the nickel or to the promoter (phosphine or amine) is at least 0.2 mol (of elementary iodine) per mol of both the nickel compound and the promoter; compare British Patent Application No. 2,007,666.
Nevertheless, the industrial-scale development of these recent techniques, the value of which is not contested in principle, seems to be jeopardized because of the relatively low efficiency of the catalyst systems in question. An additional obstacle to the development of this type of process lies in the essential use of phosphines (or amines), the instability and cost of which detract from the overall economy of the processes.